As with manufactured goods in general, the components of optical assemblies are manufactured within tolerances that allow for some variability within the completed optical assemblies of a particular nominal design due to variations in the components and in the positioning and orientation of the components with respect to one another. The effect of such variability, as deviations from an optimum design, can vary from optical assembly to optical assembly such that the optical performance of some optical assemblies can be significantly lower than the performance of other optical assemblies whose components are manufactured to the same tolerances.
Misalignment errors in optical assemblies can significantly degrade the optical performance of the optical assemblies. Such misalignments in imaging systems can produce variations in image quality across an image plane and reduce focal depth normal to the image plane. The misalignments, which generally disrupt the symmetry of the optical systems, are generally not offset by other errors in the optical systems.
In addition, misalignment errors tend to be more difficult to control than other types of errors because the misalignments generally occur at interfaces between components and their mounts. Variations in either the components or their mounts can affect the alignment of the components with respect to their mounts. Even within components and mounts manufactured to optimum nominal dimensions, clearance required for such purposes as facilitating of assembly, accommodating dimensional changes due to temperature variations, or accommodating the prescribed tolerance ranges allows for some variability in the positioning or orientation of the components within the optical assembly.
For optimum alignment, the optical axes of the components must be aligned with the alignment axis of the optical assembly. Lateral shifts of component axes with respect to an alignment axis of the optical assembly are referred to as “decenter” misalignments, and angular departures of the component axes with respect to the alignment axis are referred to as “tilt” misalignments.
Planar mounting interfaces between components and their mounts generally permit decentering of the components throughout the clearance space, such as between the outer diameters of lenses and the inner diameters of barrel surfaces within which the lenses are mounted. Mounts that engage spherical optical surfaces of the components, including spherical extensions of the optically functional zones of the optical surfaces, exploit coincidences with the mounted optical surfaces but still produce both decenter and tilt misalignments for the opposite side optical surfaces. Separately or in combination, decenter and tilt misalignments can produce a number of aberrations including astigmatic, comatic, and higher order aberrations.